A powder magnetic core obtained by binding soft magnetic powder particles with a binder such as resin is well in material yield of fabrication, as compared to the laminated magnetic core manufactured by using a silicon steel plate or the like, and it has an advantage that reduction of the material cost is possible. It has also another advantage that flexibility in shape is high and it is thus possible to improve the magnetic properties by performing optimal design of the magnetic core shape. In such a powder magnetic core, it is possible to greatly reduce the eddy current loss of the magnetic core by mixing an insulating material such as an organic binder or an inorganic powder into the soft magnetic powder, or by covering the surface of the soft magnetic powder with an electrically insulating film, so as to improve the electrical insulation between the metal powder particles.
From these advantages, the powder magnetic core is used in transformers, reactors, thyristor valves, noise filters and the like, and it is also used for an iron core of motors, a rotor or a yoke of motors in consumer electronics and industrial equipment, and it is further used for a solenoid core (fixed iron core) of an electromagnetic valve which is incorporated in an electronic controlled fuel injection system for diesel engines or gasoline engines, and the like. Thus application to a variety of soft magnetic components is progressing. As compared to the silicon steel plates, the powder magnetic core is capable of reducing the eddy current loss at high frequency range, and application of the powder magnetic core is growing in the utility for the high frequency range such as reactors and the like. Further, the higher frequency of the used frequency band allows miniaturization of the magnetic core itself and reduction of winding number and copper usage in the coil. Therefore, space saving and cost reduction of electronic devices that make use of them can be achieved. Therefore, in recent years, the used band has become higher frequency in many electronic devices and development of a material to adapt to the high frequency is rapidly progressing.
The method of forming the powder magnetic core is roughly classified into: the injection molding method for forming the soft magnetic powder by injecting it together with a raw material of plasticity into the mold that defines the product shape (Patent Literature 1, etc.); and the die compacting method in which a raw material powder containing a soft magnetic powder and a binder is filled in the die hole and compressed to be shaped by upper and lower punches (Patent Literatures 2 and 3, etc.). The product shape of the powder magnetic core is given in the die compacting process, and the forming method to adopt may vary depending on the use of product.
Under the recent demand of miniaturization and weight reduction to various types of equipment for household and industrial as described above, requirement for the powder magnetic core to improve magnetic properties such as magnetic flux density and the like has been increasing. In the powder magnetic core, since the space factor of the soft magnetic powder is proportional to the magnetic flux density, it is necessary to increase the density in order to obtain a powder magnetic core of high magnetic flux density. Therefore, as compared with the injection molding method that requires a large amount of binder, the die compacting method is widely used because it is possible to form to a higher density by decreasing the amount of binder to increase the amount of soft magnetic powder.
In the production of powder magnetic core by die compacting, a raw material powder containing the binder resin and the soft magnetic powder, or, a raw material powder consisting of soft magnetic powder having an insulating film on the surface is filled in a hole of a pressing die of the die assembly and then compressed by upper and lower punches. A specific example of the process for forming a cylindrical green compact for the magnetic core according to the die compacting method as described above is shown in FIG. 1. The die assembly shown in FIG. 1 is equipped with a die 1 having a die hole 1a for defining the outer peripheral side surface of the compact by the inner bore surface; a lower punch 2 which defines the lower surface of the compact; and an upper punch 3 which defines the upper surface of the compact. Using such a die assembly, a cavity is formed with the die hole 1a of the die 1 and the lower punch 2 and the raw material powder M is filled in the cavity by means of a powder supply device such as a feeder 4 or the like, as shown in FIG. 1(a). Then, as shown in FIG. 1(b), along with lowering the upper punch 3, the lower punch 2 is relatively raised with respect to the die 1 (in the case of this figure, the die 1 is lowered), so that the raw material powder M filled in the cavity is compressed by the upper punch 3 and the lower punch 2 to form a powder compact C. Thereafter, the upper punch 3 is moved upward to return to the standby position, and, at the same time, the lower punch 2 is relatively raised with respect to the die 1 (in the case of this figure, the die 1 is further lowered) so that the powder compact C is extracted from the die hole 1a of the die 1, as shown in FIG. 1(c).